From the discovery of the first carbon nanotubes (S. Iijima, Nature 354, 56, 1991; A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H. Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tománek, J. E. Fischer, R. E. Smalley, Science 273, 483, 1996), the generation of tubular structures with submicrometric diameters and lengths one hundred thousand times the diameter or larger awake the interest of scientist and engineers. Currently, cylindrical structures such as nanotubes, of other materials different from carbon, are ideal candidates in the development of new technological applications in fields so diverse as: field and magnetic emitting screens (N. I. Kovtyukhova, T. E. Mallouk and T. S. Mayer, Adv. Mater. 15, 780, 2003), bio-catalysis and bio-separation (D. T. Mitchell, S. B. Lee, L. Trofin, N. Li, T. K. Nevanen, H. Soderlund and C. R. Martin, J. Am. Chem. Soc. 124, 11864, 2002), drug delivery (J. M. Schnur, Science 262, 1669, 1993), adsorbents (Y. Zhang and A. Reller, Chem. Comm. 606, 2002), and even electrical energy generators from microfluidic motion (J. Yang, F. Lu, L. W. Kostiuk, D. Y. Kwok, J. Micromech. Microeng. 13, 963-970, 2003).
There is a great variety of materials to build these tubular structures. For example, Greiner's group (M. Bognitzki, H. Hou, M. Ishaque, T. Frese, M. Hellwig, C. Schwarte, A. Schaper, J. H., Wendorff, A. Greiner, Adv. Mater., 12, 9, 637-640, 2000.) developed its own process, termed TUFT, to generate nanotubes of polymeric materials, metallic and even hybrids. The technique consists on the generation, by electrodynamic means (electrospinning) of nanofibers of a certain polymer called PLA (poly(L-lactide)) and to use these nanofibers as nanotemplates. Later on, those nanofibers are coated with the desired material by means of Chemical Vapor Deposition (CVD). Once the fibers of PLA (nanotemplates) have been coated they are thermically degraded and they are extracted from the inside leaving only the tubular structure of the material which was deposited by CVD. Ai and coworkers (S. Ai et al., J. Am. Chem. Soc., 125, 11140-11141, 2003) used the wall of the cylindrical pores in a membrane as external nanotemplates. In this case, the flow of a fluid with the appropriate precursors, in specific conditions, through those pores originated certain deposition of material, layer by layer, on the wall of the pores. At certain point, the membrane is eliminated by means of an appropriate degradation, leaving only the nanotubes grown in the interior of the cylindrical pores. In general, there exist references in the literature to different processes to generate nanotubes of semiconducting, polymers, metals and other materials (S. M. Liu, L. M. Gan, W. D. Zhang, H. C. Zeng, Chem. Mater. 14, 1391, 2002; H. Q. Cao, Y. Xu, J. M. Hong, H. B. Liu, G. Yin, B. L. Li, C. Y. Tie, Z. Xu, Adv. Mater. 13, 1993, 2001; C. M. Zelenski, P. K. Dorhout, J. Am. Chem. Soc. 120, 734, 1998; C. R. Martin, Science 266 1961, 1994; V. M. Cepak, C. R. Martin, Chem. Mater. 11, 1363, 1999). However, all of these processes used solid templates to give shape to the nanotubes.
On the other hand, there also exist processes to generate nanotubes in which templates are not used. In these processes, the formation of the tubular structure is driven by exclusively chemical forces (self-assembly), but they present the disadvantage that the chemistry is very specific: the simple change of a precursor for another one, chemically very similar, disallows the process of nanotube formation (R. M. Wang, Y. J. Xing, J. Xu, D. P. Yu, New J. Phys., 5, 115, 2003; W. Chen. L. Q. Mai, Q. Xu, Q. Y. Zhu, J. F. Peng, published on web www.scipress.com/0-87849-926-1/145.htm, 2003; V. Ya. Prinz, A. V. Chekhovskiy, V. V. Preobrazhenskii, B. R. Semyagin, A. K. Gutakovsky, Nanotechnology 13, 231-233, 2002; H. Matsui, C. Holtman, Nano Lett., 2, 887, 2002).
Even in the procedures in which nanotemplates are used, it is necessary that some forces drive the molecules that makes the nanotube towards the wall of the nanotemplate and force their assembly, that is, all the methods are affected by the specific chemical components which cause the self-assembly of the proper molecules on the nanotemplate, subtracting generality to the process, although no so dramatically as in the methods which are exclusively chemical. Furthermore, the methods based on solid nanotemplates on which the molecules forming the nanotube are assembled, need multiple steps to generate the nanotube from the basic materials: (1) formation of the solid nanotemplate, (2) assembling of the structure on the nanotemplate, (3) degradation or decomposition of the nanotemplate, and (4) extraction of the nanotemplate residue. Those 4 steps are usually described as only 3 in the literature: either the formation of the nanotemplate is not considered, or steps 3 and 4 are considered as just one step. In short, in the best case, the better processes described in the literature consist at least of 3 steps.